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HI
🎉 恭喜！ 🎉
你出色地完成了任务！这是一个不小的挑战，但你找到了正确答案。你离编程大师又近了一步！继续保持，每一步都让你更强大。
课程 8. Dictionary
难度：
任务«字母的数量»
💻 Python
你在一个语言学实验室工作。你的同事，一位语言学家，需要分析文本以研究各种字符的出现频率。你的任务——开发一个程序，该程序接收一行文本作为输入，并计算每个符号在该行中出现的次数。结果必须以“符号：数量”列表的形式呈现，每对显示在新的一行上。
输入格式
输入提供一行内容
输出格式
程序应推导出“符号：数量”，其中符号 – 是输入行中的一个字符，而数量 – 是该字符在行中出现的次数。每对结果应在新的一行上输出。
示例
输入
你好，世界！
输出
h：1
e：1
l：3
o：2
，：1
& nbsp;：1
w：1
r：1
d：1
！：1
提示
Dictionaries in python
Dictionaries in Python are a powerful and indispensable data structure that allows you to store information in the form of key-value pairs. Imagine a real dictionary or phone book: knowing a unique word (key), you instantly find its definition (meaning). Dictionaries simplify organization and, most importantly, provide very fast access to data based on unique keys.
The main characteristics of dictionaries
- The keys are unique: Each key in the dictionary must be unique. Trying to add a key that already exists will simply overwrite its value.
- Changeability: Dictionaries are mutable objects. You can add, modify, and delete key-value pairs after creating a dictionary.
- Disorder (historically): In older versions of Python (before 3.7), dictionaries were considered unordered. However, starting with Python 3.7, they preserve the order in which elements are inserted. You should be careful to rely on this order if your code is supposed to run on older versions.
A useful tip: Only immutable data types such as strings, numbers, or tuples can be used as keys. Lists or other dictionaries cannot be keys, as they can be changed. But you can use absolutely any data type as values: numbers, strings, lists, and even other dictionaries.
Ways to create dictionaries
Dictionaries can be created in several ways:
1. Empty dictionary
my_dict = {}
# or using the built-in function
my_dict = dict()
2. Dictionary with elements (literal method) This is the most common and readable way.
user_profile = {
"username": "alex",
"level": 15,
"items": ["sword", "shield", "potion"]
}
3. Using the dict function with named arguments This method is convenient for simple dictionaries where the keys are strings.
my_dict = dict(key1="value1", key2="value2")
4. Creating tuples (pairs) from a list This method is useful when the data is received as a list of pairs.
pairs = [('a', 1), ('b', 2), ('c', 3)]
my_dict = dict(pairs)
# Result: {'a': 1, 'b': 2, 'c': 3}
Access to the elements
Dictionary values are accessed through their keys in square brackets.
my_dict = {"key1": "value1", "key2": "value2"}
print(my_dict["key1"]) # Output: value1
An important point: If you try to access a key that does not exist, Python will cause a KeyError error. To avoid this, use the .get() method.
Secure access using the .get() method The .get() method returns None if the key is not found and does not cause an error. You can also pass the second argument the default value, which will be returned instead of None.
print(my_dict.get("key3")) # Output: None
print(my_dict.get("key3", "not found")) # Conclusion: not found
Changing and adding elements
Adding or changing elements is very simple:
my_dict = {"key1": "old value"}
# Changing an existing element
my_dict["key1"] = "new value"
print(my_dict["key1"]) # Output: new value
# Adding a new element
my_dict["key3"] = "value3"
print(my_dict) # Output: {'key1': 'new value', 'key3': 'value3'}
To combine dictionaries, you can use the .update() method:
dict1 = {'a': 1, 'b': 2}
dict2 = {'b': 3, 'c': 4}
dict1.update(dict2) # Key 'b' will be overwritten, 'c' will be added
print(dict1) # Output: {'a': 1, 'b':3, 'c':4}
Deleting elements
There are several ways to delete items in dictionaries.:
- Using the
del operatorDeletes a key pair. Causes aKeyErrorerror if the key is not found.del my_dict["key1"] - Using the
.pop() methodDeletes an element by key and returns its value. This is useful if you need to use a deleted value. If the key is not found, it will cause aKeyError, but you can pass the default value.my_dict = {'key2': 'value2', 'key3': 'value3'} value = my_dict.pop("key2") print(value) # Output: value2 print(my_dict) # Output: {'key3': 'value3'} - Deleting all elements The
.clear()method clears the dictionary, making it empty.my_dict.clear() print(my_dict) # Output: {}
Iterating through dictionary elements
1. Key iteration This is the default behavior when iterating through the dictionary.
for key in my_dict:
print(key)
2. Iterating through values Use the .values() method if you only need values.
for value in my_dict.values():
print(value)
3. Iterating through key-value pairs This is the most efficient and readable way if you need both keys and values at the same time. Use the .items() method.
for key, value in my_dict.items():
print(f"Key: {key}, Value: {value}")
main.py
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测试 10
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